1. Field of the Invention
The present invention relates to a domestic appliance, particularly a cooking appliance, having an exposed metallic surface, for instance a stainless steel, chromed steel, nickel-chromed steel, or aluminum surface, that can be subject to mechanical (scratching), chemical (staining), and thermal (yellowing) deterioration, and that can benefit from added protection against the consequent aging during ordinary use.
2. Description of the Related Art
It is generally recognized that metallic parts used in the domestic appliance field (e.g., stainless steel, chromed-steel, nickel-chromed steel, and aluminum components such as cooktop tables, gas burners and caps, grids, oven grates, oven frontal panels, barbecue grids, etc.) are exposed to a high-temperature environment. This high-temperature can induce accelerated oxidation over some metallic surfaces. Stainless steel, for example, is thereby subject to aesthetically unpleasing yellowing. In certain cases (i.e. in gas cooking appliances) the environment is further enabling oxidation due to the presence of flames near the metal surfaces. Moreover, these parts are subject to chemical corrosion and stainability caused by contact with food-stuff and cleaning agents, and the time-consuming and abrasive efforts required to clean such surfaces (e.g., a stainless steel surface) can cause surface scratches, which then provide a rough substrate onto which charred food-spills can further attack. Furthermore, the removal of exogenous material, such as evaporated water scale residues, normally requires chemical agents creating growing environmental concerns. Moreover fingerprint marks, as well as halos and residues of detergents are very visible and not easy to remove on stainless steel components. Furthermore, aluminum components cannot be safely dish-washed without blackening their surface, because alkaline detergents present in the dishwasher soap cause chemical attack to aluminum.
Sol-gel based methods of depositing inorganic protective films have been applied in the industry, but the curing temperatures and times required to obtain sufficiently dense coatings, enabling good oxygen-barrier properties, cause yellowing of the stainless steel substrate. Moreover, the maximum coating thickness achieved by such known processes is of one or two microns, unless a multi step coating process is applied. This low thickness can reduce the oxygen barrier performance, compared to thicker (e.g., 5 microns) but equally or more dense coatings made of the same material.
U.S. Pat. No. 5,192,410 discloses a method for coloring a stainless steel parts of buildings and automobiles in which a colored and transparent double coating is envisaged. Such patent is not concerned with resistance at high temperatures (typical for cooking appliances), rather with rust resistance only (weather), i.e. corrosion at ambient temperature.
JP 62 087114 is concerned with coating cooking utensils (i.e. frying pans, hot plates etc.) with SiO2 in order to increase the cooking efficiency of food.
JP 61 110769 relates to the coating of knife blade edges with Si oxides to avoid contamination of the cut food by metallic salts and ions otherwise leached out from the steel blade.
Accordingly, it would be advantageous to provide metallic parts used in the domestic appliance field with improved protection against high-temperature oxidation, corrosion, abrasion, and maintenance wear-and-tear. A secondary objective is to provide stainless steel parts used over domestic appliances, particularly cooking appliances, with a lower visibility of stains/fingerprints and an easier, less time consuming, and more environmental friendly removal of detergent residual halos, water-induced scale, and fingerprints when compared to bare stainless steel surfaces. A further objective is to enable dishwashing possibility (i.e., alkaline corrosion resistance) for aluminum components.